Polycarbonates, especially, aromatic polycarbonates have been widely used in many fields as engineering plastics excellent in heat resistance, impact resistance, transparency, and the like.
As a method of producing polycarbonate, known are the following methods for example:
(i) a method of causing interfacial polycondensation of bisphenol A with phosgene in the presence of an alkali catalyst (phosgene method); and
(ii) a method of causing melt-polycondensation of bisphenol A with diphenyl carbonate (ester exchange method).
In the method (i), colorless transparent polycarbonate can be obtained since the reaction proceeds at a low temperature. However, the method (i) has the following problems.
Toxic phosgene is used.
Impurities such as sodium chloride which is produced as a by-product by the reaction are not easily removed.
In the method (ii), polycarbonate is easily separated from reaction system since a solvent does not need to be used. However, the method (ii) has the following problems.
The reaction rate of an ester exchange reaction of bisphenol A and diphenyl carbonate is low and polycondensation needs to be carried out at a high temperature for a long period of time.
The boiling point of phenol to be separated by the ester exchange reaction is high and thus, the removal thereof requires a high temperature.
A side reaction or the like occurs during polycondensation due to the high temperature and polycarbonate is colored.
Phenol which is generated by the ester exchange reaction is difficult to remove because of high viscosity of a product and thus, a high molecular weight substance is unlikely to be obtained.
As a method of improving the problems of the method (i) or the method (ii), the following method (iii) is suggested (see PTLs. 1 and 2):
(iii) a method which includes a process of reacting an aromatic dihydroxy compound and diaryl carbonate to obtain a prepolymer; a process of crystallizing the prepolymer to obtain a crystallized prepolymer; and a process of heating the crystallized prepolymer in a temperature of from the glass transition temperature to lower than the melting temperature of the crystallized prepolymer and performing solid phase polymerization on the crystallized prepolymer to obtain a polycarbonate (solid phase polymerization method).
In the method (iii), phosgene does not need to be used. Further, since a polycarbonate with a high molecular weight can be produced at a low temperature as compared to the method (ii), coloration is not a problem. However, in the method (iii), a prepolymer needs to be crystallized in advance because a prepolymer which is not crystallized does not undergo the solid phase polymerization. In order to crystallize a prepolymer, it is necessary to perform any of operations of dissolving the prepolymer in a solvent and allowing it to be deposited to be crystallized; infiltrating a solvent whose dissolving power with respect to the prepolymer is weak into the prepolymer; and heating and maintaining the prepolymer in a temperature of from the glass transition temperature to lower than the melting temperature. Consequently, the method (iii) has a problem in that the process is complicated and the productivity is degraded.